In extracorporeal blood processing, blood is taken out of a human subject, processed (e.g. treated) and then reintroduced into the subject by means of an extracorporeal blood flow circuit (“EC circuit”) which is part of a blood processing apparatus. Generally, the blood is circulated through the EC circuit by a blood pump. In certain types of extracorporeal blood processing, the EC circuit includes an access device for blood withdrawal (e.g. an arterial needle) and an access device for blood reintroduction (e.g. a venous needle), which are inserted into a dedicated blood vessel access (e.g. fistula or graft) on the subject. Such extracorporeal blood processing includes hemodialysis, hemodiafiltration, hemofiltration, plasmapheresis, etc.
In extracorporeal blood processing, it is vital to minimize the risk for malfunctions in the EC circuit, since these may lead to a potentially life-threatening condition of the subject. Serious conditions may e.g. arise if the EC circuit is disrupted downstream of the blood pump, e.g. by a VND event, in which the venous needle comes loose from the blood vessel access. Such a disruption may cause the subject to be drained of blood within minutes.
VND may be detected during blood processing based on a pressure signal from a pressure sensor (“venous pressure sensor”) on the downstream side of the blood pump in the EC circuit. Conventionally, VND monitoring is carried out by comparing one or more measured pressure levels with one or more threshold values. However, it may be difficult to set appropriate threshold values, since the pressure in the EC circuit may vary between treatments, and also during a treatment, e.g. as a result of the subject moving. Further, if an access device comes loose and gets stuck in bed sheets or the subject's clothes, the measured pressure level might not change enough to indicate the potentially dangerous situation.
WO97/10013 proposes alternative techniques for VND monitoring based on the pressure signal measured by the venous pressure sensor in the EC circuit. In one alternative, VND monitoring is based on detection of heart pulses in the pressure signal. The heart pulses represent pressure pulses produced by a patient's heart and transmitted from the patient's circulatory system to the venous pressure sensor via the blood vessel access and the venous needle. In an alternative, VND monitoring is based on pressure pulses (pump pulses) that are generated by the blood pump and transmitted from the blood pump to the venous pressure sensor via the arterial needle, the blood vessel access and the venous needle. An absence of pump pulses in the pressure signal thereby indicates that the arterial needle and/or the venous needle is dislodged.
US2005/0010118, WO2009/156174 and US2010/0234786 disclose similar or alternative techniques of VND monitoring based on detection of heart pulses in the pressure signal acquired from a venous pressure sensor.
WO2010/149726 discloses techniques for VND monitoring based on detection of physiological pulses other than heart pulses in the pressure signal from the venous pressure sensor. Such physiological pulses originate from the human subject, e.g. from reflexes, voluntary muscle contractions, non-voluntary muscle contractions, the breathing system, the autonomous system for blood pressure regulation or the autonomous system for body temperature regulation.
The prior art also comprises WO2009/127683, which discloses a technique for VND monitoring, by isolating a beating signal in the pressure signal obtained from the venous pressure sensor. The beating signal manifests itself as an amplitude modulation of the pressure signal and is formed by interference between pressure waves generated by a patient's heart and pressure waves generated by the blood pump. An absence of the beating signal indicates that the venous needle is dislodged.
In certain configurations or operating conditions of the EC circuit, the pressure waves generated by the subject's heart or another physiological phenomenon in the human subject may be too weak to be reliably detected in the pressure signal of the venous pressure sensor. Thus, many of the above techniques may be unreliable in these configurations/operating conditions.
Furthermore, there are blood treatment apparatuses that have no venous pressure sensor, or where the venous pressure sensor has a design or placement that does not allow reliable detection of physiological pulses/pump pulses.
There is thus a need for an alternative or supplementary technique for VND monitoring in EC circuits.